Carbon contained hard masks, such as amorphous carbon layers (ACL) or pattern definition film (PDF), have been widely adapted for use as masking material for etching high aspect ratio structures, such as DRAM or 3D NAND devices (e.g., for applications such as OCS etch in DRQAM or high aspect ratio contact etch in 3D NAND). Etching or stripping processes with high aspect ratio structures can pose several challenges.
For example, a substrate can include a patterned area with high aspect ratio structures and an unpatterned area without high aspect ratio structures. A strip process performed after etching can require removal of a carbon hard mask in both the patterned area and the unpatterned area before wet clean. However, the remaining mask after etch in the patterned area can be much thinner than the unpatterned area due to, for instance, pattern proximity effect and stronger mask erosion from ion bombardment dielectric plasma etch, etc. As a result, the patterned area can experience much longer over strip than the unpatterned area during processes intended to remove the carbon mask cleanly everywhere on the substrate.
It is increasingly becoming required to have higher mask selectivity during etch processes of high aspect ratio structures. Accordingly, dopant (e.g., boron) can be added into the carbon mask materials to create doped amorphous carbon (DaC) films to enhance mask selectivity. However, with traditional strip processes, the mask removal rate of the DaC films has been much lower than the removal rate that can be reached with conventional amorphous carbon film using traditional strip processes.